In the past, microstructured materials have been obtained by methods of rapidly solidifying composite metal(s), alloy(s), compound(s), etc., whereby most of the obtained microstructured materials have particle sizes of several microns.
In recent years, research has been actively conducted seeking to minimize the particle size, i.e., from the micron to nanometer order. One of the primary characteristics of nanostructures containing such nanoparticles is that a high percentage of atoms exist on the particle boundary (surface), for example, the ratio may reach 40% with nanoparticles having a diameter of 5 nm. Nanostructured materials may have chemical and physical characteristics that differ greatly from those of micro-level materials having the same chemical composition, and nanostructured materials often exhibit more desirable characteristics.
Nanostructured materials, which have a large surface area, are particularly useful for applications in which a chemical reaction mediated by an active center plays a significant role, i.e., catalytic applications. The larger the contact area such materials have with the surroundings (gases, liquids, etc.), the better the catalytic reaction should be. Furthermore, when a transition metal nanostructured material is used as a material having a large surface area, by giving or receiving electrons to or from the reaction material, the transition metal itself is subject to valence change (oxidation/reduction), so that great catalytic activity can be thereby readily obtained. Therefore, there is a clear advantage in forming an active layer for an electrochemical electrode having catalytic activity from a transition metal nanostructured material.
Examples of known transition metals that can be used as an active layer of an electrochemical electrode include nickel, manganese, etc. For example, Patent Document 1 discloses a fuel cell electrode in which nickel nanoparticles supported on carbon particles are used as an electrode material.
Patent Document 2 discloses a zinc air battery, wherein a micron-level powder mixture of trimanganese tetroxide and manganese dioxide is used as an oxygen reduction electrode. The invention of Patent Document 2 aims to improve the stability and catalytic activity of the oxygen reduction electrode by increasing the efficiency of electron transfer (oxidation/reduction) by using a plurality of manganese oxides having different valences in combination.
An example of a method for making a transition metal into nanoparticles includes, when nickel-nanoparticles disclosed in Patent Document 1 are used, nickel hydroxide, which is more stable than nickel, is made into nanoparticles, supported on a carrier, and then reduced to nickel.
In addition to the Patent Documents mentioned above, Patent Documents 3 to 4 below are relevant to the present invention.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-223130 (page 6, FIG. 1)
[Patent Document 2] Japanese Unexamined Patent Publication No. 10-302808 (page 8, FIG. 2)
[Patent Document 3] WO No. 2005/019109
[Patent Document 4] Japanese Unexamined Patent Publication No. 2004-286466